Updates Travis golang version to latest stable, which might also get rid of 'package math/bits: unrecognized import path math/bits' warning from promtool install.
Updates Travis golang version to latest stable, which might also get rid of 'package math/bits: unrecognized import path math/bits' warning from promtool install.

stephen-soltesz/prometheus-support,stephen-soltesz/prometheus-support,stephen-soltesz/prometheus-support